1. Field of the Invention
The present invention generally relates to a transport apparatus, and in particular to a transport apparatus for transporting a planar, conductive object without contacting the object.
2. Discussion of the Related Art
In the photolithography step of manufacturing semiconductor devices, for example, an exposure apparatus is used. This apparatus has been used for transferring through a projection optical system a circuit pattern formed on a mask (which will be generally called "reticle") onto a photosensitive substrate, such as a wafer, whose surface is coated with photoresist. The wafers to be exposed are successively transported by a transport apparatus to a predetermined position on an exposure stage of the exposure apparatus.
An example of the transport apparatus is a vacuum hand that applies a vacuum suction to a wafer. This vacuum suction holds and transports the wafer. FIG. 8 is a plan view showing a conventional vacuum hand for transporting a wafer. FIG. 9 is a front view of the conventional vacuum hand of FIG. 8. The vacuum hand 102 is provided with a vacuum groove 104 formed in an upper surface of a distal end portion thereof to suck the wafer by a vacuum force generated by a vacuum pump (not shown) and applied through the vacuum groove 104. To transport the wafer W, therefore, the distal end portion of the vacuum hand 102 having the vacuum groove 104 is brought into contact with the rear surface of the wafer W, and a vacuum force is applied to the wafer W through the groove 104 to capture the wafer W on the vacuum hand 102. In this condition, the vacuum hand 102 is moved to an unloading position on the exposure stage, and then the suction is stopped so that the wafer W is released from the vacuum hand 102. In this manner, the wafer W is transported to a predetermined position on a wafer stage. Thus, the vacuum hand 102 transports the wafer W while contacting the rear surface of the wafer W.
Another example of the transport apparatus is an electrostatic floating type, which utilizes electrostatic force generated by electrodes. The electrostatic floating type apparatus captures the wafer in the air by balancing the attraction force with the self-weight of the wafer (FIG. 10). In this electrostatic floating type transport apparatus, a large number of electrode portions 108 are arranged in a bottom surface of a main body 106 of the apparatus at a regular interval, for example, and an electrostatic potential (voltage) having a predetermined polarity is applied to each of the electrode portions 108.
In transporting the wafer W using this electrostatic floating type transport apparatus, positive (+) potentials are applied to the electrode portions 108 located right above the wafer W (five electrode portions in FIG. 10) and negative (-) charges are induced on the surface of the wafer W facing the electrode portions 108 with a clearance (corresponding to dielectric) due to electrostatic induction of the wafer W (conductive body) doped with ions. Therefore, the wafer W is attracted toward the main body 106 (in the direction of arrow "C" in FIG. 10) due to the electrostatic force acting between different kinds of charges. The potential applied to each of the electrode portions 108 is controlled so as to balance the self-weight of the wafer W with the generated electrostatic force (attraction force). Thus, the wafer W is kept floating in the air, maintaining a predetermined distance from the main body 106, as shown in FIG. 10. If the main body 106 of the transport apparatus is moved in this situation, the wafer W is transported without contacting the main body 106.
To control the position of the floating wafer W with respect to the main body 106, a positive (+) potential is applied to an electrode portion 108a located at the right end of the wafer W of FIG. 10, and a negative (-) potential is applied to an electrode portion 108b located at the left end of the wafer W, for example. With negative (-) charges induced in the surface of the wafer W, the electrode portion 108b and the wafer W repulses from each other, and the electrode portion 108a and the wafer W are attracted to each other, thereby moving the wafer W to the right (in the direction of arrow "D" in FIG. 10). Thus, in the electrostatic floating type apparatus, the horizontal position of the wafer W in the two-dimensional plane relative to the main body 106 can be finely adjusted by controlling the potentials of the electrode portions 108 located at the end portions of the wafer.
When the above-described vacuum hand 102 is used to transport the wafer W, dust or other foreign objects tend to stick to the wafer W due to contact between the vacuum band 102 and the rear surface of the wafer W; this dust or the like degrades the yield in manufacture of semiconductor devices.
In the electrostatic floating type transport apparatus in which electric potentials are applied to the electrode portions 108 to produce electrostatic force for attracting the wafer W while keeping it floating, on the other hand, dust or the like is unlikely to stick to the wafer since the wafer and the transport apparatus do not contact each other; the problem of poor yield is therefore prevented. However, since the wafer W is held in the air without contacting the transport apparatus and almost no friction is present between the wafer W and the transport apparatus, it is difficult to stop rotation of the wafer W. It also is difficult to control the rotational angle .theta. of the wafer W if rotational momentum about the Z-axis arises due to some disturbance applied to the wafer W during the movement of the main body 106 or during the position control of the wafer W relative to the main body 106, for example. Therefore, the wafer W can not be placed in a predetermined (or design) position on the wafer stage with high accuracy.